The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 2000-296970 filed Sep. 28, 2000; and
Japanese Patent Application No. 2000-296976 filed Sep. 28, 2000.
1. Field of the Invention
The present invention relates to a zoom lens system and, in particular, relates to a zoom lens system having a high zoom ratio.
2. Description of Related Art
Since degree of freedom for optical design has been increased with recent progress in manufacturing technology regarding an aspherical lens and a lens barrel, a zoom lens system having a high zoom ratio achieved by using a well-known positive-negative-positive-positive four-lens-group type has been proposed. The positive-negative-positive-positive four-lens-group type is composed of, in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power.
Specifically, it has been disclosed in Japanese Patent Application Laid-Open No. 8-94933 and in Japanese Patent Application Laid-Open No. 11-30750.
In Japanese Patent Application Laid-Open No. 8-94933, a zoom lens system having a zoom ratio of about five is achieved by arranging an aspherical plane in a second lens group with increasing an angle of view in the wide-angle end state. In Japanese Patent Application Laid-Open No. 11-30750, a zoom lens system having an increased zoom ratio achieved by arranging a large number of aspherical surfaces is proposed. Moreover, in Japanese Patent Application Laid-Open No. 11-84202, a lens barrel suitable for a zoom lens having a zoom ratio of ten is proposed.
However, in a conventional positive-negative-positive-positive four-lens-group type, a high zoom ratio and compactness have been achieved by increasing refractive power of each lens group, so that optical performance has been greatly degraded by assembling error produced while manufacturing. Moreover, the number of lenses consisting of each lens group becomes large, so that transportabilIty cannot be improved. Furthermore, variation in off-axial aberrations produced while changing the lens group positions is difficult to correct well.
However, in a conventional positive-negative-positive-positive four-lens-group type, when increasing a zoom ratio, transportability has been liable to be spoiled by the reasons such that the lens diameter becomes large or that the total lens length cannot be shorten.
The present invention is made in view of the aforementioned problems and has an object to provide a zoom lens system with a zoom ratio of seven or more suitable for compactness capable of obtaining stable optical performance in spite of reducing the number of lenses.
According to one aspect of the present invention, a zoom lens system includes, in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power. Each lens group is moved to the object side such that when the state of lens group positions of the zoom lens system is changed from a wide-angle end state to a telephoto end state, the separation between the first lens group and the second lens group increases, the separation between the second lens group and the third lens group decreases, and the separation between the third lens group and the fourth lens group decreases. An aperture stop is arranged in the vicinity of the third lens group. The fourth lens group is composed of four lens elements arranged, in order from the object, a first positive lens element, a first negative lens element, a second positive lens element, and a second negative lens element.
In one preferred embodiment of the present invention, the first positive lens element has a double convex shape and the first negative lens element has a meniscus shape having a convex surface facing to the object side.
In one preferred embodiment of the present invention, the following conditional expression is satisfied:
0.12 less than r41/r42 less than 0.45xe2x80x83xe2x80x83(1)
where r41 denotes the radius of curvature to the object side of the first positive lens element arranged to the most object side of the fourth lens group, and r42 denotes the radius of curvature to the image side of the first negative lens element arranged second in order from the object side of the fourth lens group.
In one preferred embodiment of the present invention, the second positive lens element and the second negative lens element are cemented together forming a cemented lens having positive refractive power.
In one preferred embodiment of the present invention, at least one of the following conditional expressions is satisfied:
0.15 less than D1/ft less than 0.25xe2x80x83xe2x80x83(2)
0.15 less than |f2|/(fwxc2x7ft)xc2xd less than 0.25xe2x80x83xe2x80x83(3)
where D1 denotes the separation along the optical axis between the first lens group and the second lens group in the telephoto end state, ft denotes the focal length of the zoom lens system as a whole in the telephoto end state, f2 denotes the focal length of the second lens group, and fw denotes the focal length of the zoom lens system as a whole in the wide-angle end state.
In one preferred embodiment of the present invention, the following conditional expression is satisfied:
0.5 less than d2/d1 less than 2xe2x80x83xe2x80x83(4)
where d2 denotes the separation between the first negative lens element arranged second in order from the object side of the fourth lens group and the second positive lens element arranged to the image side of the first negative lens element, and d1 denotes the separation between the first positive lens element arranged most object side of the fourth lens group and the first negative lens element arranged to the image side of the first positive lens element.
According to another aspect of the present invention, a zoom lens system includes, in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power; and a fourth lens group having positive refractive power. Each lens group is moved to the object side such that when the state of lens group positions of the zoom lens system is changed from a wide-angle end state to a telephoto end state, the separation between the first lens group and the second lens group increases, the separation between the second lens group and the third lens group decreases, and the separation between the third lens group and the fourth lens group decreases. The second lens group is composed of four lens elements arranged, in order from the object, a negative lens having a concave surface facing to an image side, a double concave negative lens, a double convex positive lens, and a negative lens having a concave surface facing to the object side. The following conditional expressions are satisfied:
3 less than |f2B|/|f2| less than 5xe2x80x83xe2x80x83(5)
0.3 less than f2P/|f2B| less than 0.45xe2x80x83xe2x80x83(6)
where f2B denotes the focal length of the three lenses in the second lens group having four lens elements arranged to the image side in the second lens group, f2 denotes the focal length of the second lens group, and f2P denotes the focal length of the double convex positive lens arranged in the second lens group.
In one preferred embodiment of the present invention, the following conditional expression is satisfied:
0.14 less than |f2|/(fwxc2x7ft)xc2xd less than 0.24xe2x80x83xe2x80x83(7)
where ft denotes the focal length of the zoom lens system as a whole in the telephoto end state, and fw denotes the focal length of the zoom lens system as a whole in the wide-angle end state.
In one preferred embodiment of the present invention, an aperture stop is arranged between the second lens group and the third lens group. In this case, it is preferable that the aperture stop is moved with the third lens group in a body while changing the state of lens group positions.
In one preferred embodiment of the present invention, the first lens group is composed of three lenses arranged, in order from the object, a negative lens having a convex surface facing to the object side, a first positive lens having a convex surface facing to the object side, and a second positive lens having a convex surface facing to the object side. The following conditional expression is satisfied:
2 less than f13/f1 less than 4xe2x80x83xe2x80x83(8)
where f13 denotes the focal length of the positive lens arranged to the image side in the first lens group having two positive lenses, and f1 denotes the focal length of the first lens group.
In one preferred embodiment of the present invention, the first lens group is composed of, in order from the object, a cemented lens constructed by a negative meniscus lens having a convex surface facing to the object side and a positive lens having a convex surface facing to the object side together, and a positive meniscus lens having a convex surface facing to the object.
In one preferred embodiment of the present invention, the following conditional expression is satisfied:
0.45 less than D2W/D1T less than 0.60xe2x80x83xe2x80x83(9)
where D2W denotes the separation along the optical axis between the second lens group and the third lens group in the wide-angle end state, and D1T denotes the separation along the optical axis between the first lens group and the second lens group in the telephoto end state.